Optical subassemblies often include optical elements bonded to supporting substrates. Many of these supporting substrates are made from silicon and are therefore, called silicon optical benches. The optical elements are typically mounted in a retaining cavity defined in the working surface of the silicon optical bench. The retaining cavity holds each optical element in precise alignment with the other components of the subassembly. The surface of the retaining cavity may include a layer of metal such as aluminum, which is used for metal oxide bonding the optical element in the retaining cavity.
Optical components are typically bonded in position by aluminum oxide bonding. Aluminum oxide bonding involves applying heat and pressure at the interface of the optical element and the aluminum layer. The heat and pressure, in the presence of aluminum, causes a silicon dioxide layer applied to the surface of the optical element to be reduced to aluminum oxide. The aluminum oxide bonds the optical element in the cavity.
The retaining cavity, the aluminum layer and the metalized regions of the silicon optical bench are fabricated with photolithographic techniques that utilize electrolytic chemicals. These methods involve coating the silicon optical bench with layers of photoresist. The photoresist may be patterned into a mask whose shape allows the cavity, the aluminum layer and the metalized regions to be fabricated in the pattern defined by the photoresist mask. After a feature is created, the photoresist mask is removed with a photoresist stripping solution. Many layers of photoresist may be deposited, patterned and stripped during the fabrication of a typical optical subassembly. Many of these photoresist stripping solutions are electrolytic in nature.
A difficulty which is commonly arises is that unwanted electrolytic reactions on the silicon optical bench weaken the strength of the aluminum oxide bond. Since aluminum is anodic (positive voltage), relative to gold, platinum, titanium, and tin, a potential drop of approximately 2.5-3 volts across the metalizations and the aluminum layer coating the cavity have been observed when the optical benches are immersed in photoresist stripping solutions. This leads to a electrochemical reaction that significantly increases the thickness of the native aluminum oxide film already present on the aluminum surface. The native aluminum oxide films are typically 20 to 35 angstroms thick and result from exposure to air. Aluminum oxide films created by anodic oxidation of the aluminum when photoresist strippers are used typically have a thickness of about 100 angstroms. Optical elements that are aluminum oxide bonded to aluminum with native aluminum oxide films exhibit acceptable optical element shear strengths of approximately 200 grams. However, optical elements aluminum oxide bonded to aluminum with aluminum oxide films thickened by anodic oxidation of the aluminum exhibit unacceptably low optical element shear strengths of approximately 10 grams.
Accordingly, there is a need for a method of preparing or pre-treating a supporting substrate of an optical subassembly in order to substantially improve the shear strength of a metal oxide bonded optical element.